Magnetic apparatus for producing a uniform field



June 28, 1966 U. A. PEURON ETAL MAGNETIC APPARATUS FOR PRODUCING A UNIFORM FIELD Filed Nov. 2?, 1965 2 Sheets-Sheet l WITNESSES INVENTORS Unro A. Peuron and obert Span BY 1 '(k1) ATTORNEY J1me 1966 u. A. PEURON ETAL 3, 7

2 Sheets-Sheet 2 Fi g.5A. F i 9.58.

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United States Patent 3,258,657 MAGNETIC APPARATUS FOR PRODUCING A UNIFORM FIELD Unto A. Peuron, Pittsburgh, and Robert E. Span, Rector,

Pa., assignors to Westinghouse Electric Corporation,

Pittsburgh, Pa., a corporation of Pennsylvania Filed Nov. 27, 1963, Ser. No. 326,552 6 Claims. (Cl. 317-158) This invention relates in general to magnetic apparatus, and more particularly to superconducting electromagnets.

As will be understood by those skilled in the art, it is desirable to have cross-field electromagnets of long working volume to assist in the operation of such scientific devices as masers. Such devices require extreme uniformity in the magnetic field if they are to function properly. The electromagnets used to provide the magnetic field may be either conventional or superconducting.

One approach used in the prior art is illustrated by US Patent 3,098,181 to Paul P. Ciofii. Ciofli uses superconducting magnetic shields to prevent leakage and fring-- ing of magnetic flux and thereby creates a magnetic field of the required uniformity and intensity.

We have found that superconducting magnetic shields have a disadvantage at the present state of the art because their usefulness is restricted to relatively small magnetic field intensities. If the critical magnetic field of the superconducting material is exceeded the shielding effect is destroyed.

Accordingly, it is an object of this invention to provide a new and improved magnet.

It is a more particular object of this invention to provide a new and improved magnet which contains the magnetic field without the use of magnetic shields.

It is yet another object of this invention to provide an electromagnet with a long transverse magnetic field of extreme uniformity.

Other objects of our invention will in part be obvious and will in part appear hereinafter.

Briefly, the present invention accomplishes the above cited objects by providing two parallel smooth surfaces of a material which will easily carry magnetic flux on either side of the source of a magnetic field. These surfaces are linked by other surfaces of a material which will easily carry magnetic fiux to make a closed magnetic circuit. We shall hereafter refer to thees surfaces as magnetic surfaces and to the members of which they are a part as magnetic members.

It is well known that a solenoid of infinite length will produce a uniform magnetic field because of the ab sence of fringing and other end effects which are necessary concomitants to a solenoid of finite length.

It is also well known that when two mirrors face each other in parallel spaced relationship that multiple reflections between the mirrors produce the effect of mirrors having an infinite depth.

We use the parallel surfaces as magnetic mirrors placed as close as possible to the source of magnetic field. Fringing of the magnetic field is prevented because the magnetic field between the magnetic mirrors can apparently extend to infinity into the parallel magnetic mirrors. Using solenoids of finite length in conjunction with the parallel magnetic mirrors, the equivalent of solenoids of infinite length is obtained. The solenoids may have any cross-section and are not limited to round or rectangular cross-sections.

Further objects and advantages of the invention will become apparent as the following description proceeds and features of novelty which characterize the invention will be pointed out in particularity in the claims annexed to and forming a part of this specification.

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For a better understanding of our invention reference may be had to the accompanying drawings in which:

FIG. 1 is a transverse sectional view of one embodiment of our invention;

FIG. 2A is an end view of a second embodiment of our invention;

FIG. 2B is a side elevation of the embodiment of our invention illustrated in FIG. 2A;

FIG. 3A is an end elevation of the embodiment of our invention shown in FIG. 1;

FIG. 3B is a side elevation of the embodiment of our invention illustrated in FIG. 3A;

FIG. 4 is a transverse sectional view of a cylindrical embodiment of our invention;

FIG. 5A is a partial transverse sectional view of an idealized solenoid or coil adjacent to a flat plane surface;

FIG. 5B is a partial transverse sectional view of the actual coil of FIG. 5A after a compensating turn has been added;

FIG. 6 is a transverse sectional view of a coil enclosed by a magnetic structure.

We refer now to the drawnigs, in which like reference numerals are used throughout to designate like parts, for a more detailed understanding of the invention. In par ticular, FIG. 1 of the drawings shows two spaced apart parallel magnetic members and 32 which may be made of iron. These magnetic members are connected by the magnetic members 38 and 40; the magnetic members 38 and 40 are held together by fastening means 34. A coil of wire 42 is placed around the magnetic member 38; the ends of coil 42 are in contact With the inner surfaces 31 and 33 of the magnetic members 30 and 32. In like manner, the magnetic member 40 is surrounded by a coil 44. When the coils 42 and 44 are energized by a direct current, the magnetic fields they produce are additive in zone 46 between the coils. Nonmagnetic spacers 36 are used inside of the magnetic members 38 and 40. These spacers may be roughened to permit a bonding agent such as epoxy cement to bond the coils to the spacers. It is not desirable to roughen the surfaces of the magnetic members 38 and 40 because of the adverse effect on the uniformity of the magnetic circuit. All embodiments of our invention have at least two parallel facing sides in the magnetic circuit such as surfaces 31 and 33. These surfaces are the main reflecting surfaces for the magnetic mirror effect.

FIGS. 2A and 2B illustrate an embodiment of our invention in which two half coils 12 and 14 are placed inside a magnetic structure rather than around a magnetic structure. A magnetic structure 10 is provided similar to the magnetic structure described in FIG. 1. The two half coils 12 and 14 are placed in thespace 16 enclosed by the magnetic structure 10; it will be noted that both half coils 12 and 14 extend beyond the ends of the magnetic structure 10. External to the magnetic structure 10 the half coils 12 and 14v are bent apart to permit a workpiece such as a maser to be inserted in the volume enclosed by the coil halves. The coil halves are energized with direct current in such a way that the magnetic field produced by each coil half is additive to the magnetic field produced by the other coil half.

In FIGS. 3A and 3B there are illustrated views of an embodiment of our invention similar to FIG. 1 with electrical and magnetic polarities as indicated. A magnetic structure 18 is provided with legs 26 and 28; about each leg is placed an electrical coil half such as electrical coil halves 20 and 22. The legs 26 and 28 are connected by orthogonal yoke members 19 and 27 the inner faces of which serve to create the effect of a coil of great length. The polarity of the direct electrical current in each coil half is indicated by dots and crosses such as dot 21 and cross 23. It will be understood that a dot indicates direct current coming out of the drawing and that a cross indicates direct current going into the drawing. This arrangement of polarities in each coil half produces a magnetic field in the working volume 24 of the magnet between the coil halves 20 and 22 which has a direction indicated by the arrow 25. Access may be had to this traverse magnetic field from either end of the magnetic structure. Two coil halves are used, rather than one large coil, to provide this access to the magnetic field.

In FIG. 4 is illustrated an embodiment of our invention as an iron bound magnet which produces a high uniform magnetic field in a cylindrical region. This configuration is desirable to produce a uniform magnetic working environment for such devices as cavity masers. The iron structure 48, as in all embodiments of our invention, has two parallel spaced apart planes 49 and 51 which are the reflecting surfaces. Between these parallel planes are placed two spaced apart toroidal coil halves 50 and 52. When these coil halves are energized by direct current a cylindrical magnetic field of great uniformity is produced in region 60. Each of the coil halves is compensated for the gap 58 between the coil halves by the addition of extra turns 54 located near the gap 58 between the coil halves 50 and 52. It is desirable to have these compensating turns approximate in crosssectional area the crosssectional area of the gap 58 between the coil halves. Additional compensating turns such as turn 56 may be used to compensate for unavoidable coil non-uniformities when round wire is used as is best illustrated in FIGS. A and 5B.

Referring to FIGS. 5A and 5B one may see when round wire 64 is used next to a flat surface '62 that, because of the shape of the wire, in every other layer of wire a half turn such as half turns 61 and 63 will be lost. These lost half turns are compensated for in FIG. 5B by the addition of a Whole turn 66 for each two lost half turns. This compensating turn 66 corresponds to the compensating turns shown in FIG. '4 such as turn 56.

At FIG. 6 there is illustrated a magnetic structure 88 and a coil 68 to show how the phenomenon of images prevents end effects in a magnetic solenoid of finite length. The drop in magnetic potential across the air gap 90 is much greater than the drop around the magnetic structure 88, hence the magnetic structure may be considered as having magnetically equi-potential surfaces. The physical coil 68 will see no discontinuities at top and bottom because an upper virtual coil 70 and a lower virtual coil 72 are produced by the magnetic field of the physical coil working into a ferromagnetic structure 88 of negligible reluctance compared to the reluctance of the air gap 90. As far as the physical coil 68 is concerned,

these virtual coils extend to infinity in both directions,

hence, no magnetic end effects are produced at the ends of the physical coil 68.

In operation, all embodiments of our invention utilize two smooth spaced apart magnetic surfaces as mirrors to give the benefits of an infinitely long coil to a coil of finite length. A coil or solenoid of great length is not only expensive and space consuming but also has no direct access to its center. Accordingly, in all embodiments of our invention we employ a split or two part coil to make access to the magnetic field simple. The parallel magnetic surfaces which enclose the half coils of each embodiment of our invention for ail practical purposes may be considered as 'equipotential magnetic surfaces. The magnetic potential drop in the magnetic members connecting the main reflecting surfaces may be consid ered zero for all practical purposes. Since the magnetic surfaces are exactly parallel, so are all the intermediate equipotential surfaces between them in the zone between the magnetic surfaces. In other words, every ampereturn produced by the split coils is consumed at the same height coordinate where it is generated. The wires used in the coils are of a very small cross-section which insures an even current distribution. Fine wires are also desirable when our invention is operated as a superconducting magnet because the thin wires have a high thermal impedance which slows down heat transfer. Each half coil is wound with the same number of turns as its corresponding half coil. All the magnetic members that we use have plane surfaces which means that the surfaces can be easily ground or lapped to the required smoothness without great diificulty. This is contrasted to any magnetic structure which has stepped cuts. The magnetic air gap is accurately determined by the legs or side spaces which not only determine the air gap but also act as magnetic return paths. The coil halves in the illustrated embodiments of our invention have a uniform rectangular cross-section. This makes it easy to space the coil halves equidistantly. However, as previously mentioned, other coil cross-sections may be used.

The nonmagnetic spacers such as shown at 36 in FIG. 1 may be roughened by the use of slots or holes so that the coils can be bonded to the spacers. As previously ientioned, it would be undesirable to roughen the magnetic surfaces by means of slots or holes to achieve this bonding because the magnetic surfaces must be maintained smooth to avoid adverse effects on the magnetic field.

It will, therefore, be apparent that there has been disclosed an electromagnet suitable for operation as a superconducting solenoid which produces a magnetic field of extreme uniformity.

Since numerous changes may be made in the abovedescribed apparatus and different embodiments may be made without departing from the spirit thereof it is intended that all the matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

We claim as our invention:

1. A cross field electromagnet comprising two spaced apart leg members, said leg members having parallel facing sides, said leg members connected by two parallel yoke members to form a closed magnetic circuit, a rectangular electrical coil wound about each of said leg members and in contact with each of said yoke members, each of said electrical coils having a generally rectangular cross-section, said coils being wound so that the magnetic flux produced by each coil between said yoke members is additive.

2. A magnet for providing a uniform magnetic field comprising first and second magnetic members each having a flat magnetic reflecting surface,

said first and second magnetic members being disposed in spaced relation with their flat reflecting surfaces in adjacent, spaced, parallel relation,

third and fourth magnetic members connecting said first and second magnetic members to form a closed magnetic circuit which defines an opening of predetermined size,

a source of magnetic flux having at least two flat parallel sides,

said source of magnetic flux being disposed between said first and second magnetic members with at least a portion of its two flat parallel sides in contact with the flat magnetic reflecting surfaces of said first and second magnetic members,

said source of magnetic flux providing a magnetic field which extends from the reflective surface of said first magnetic member to the reflective surface of said second magnetic member, and which returns from the second magnetic member to the first magnetic member through said third and fourth magnetic members.

3. The magnet of claim 2 wherein said source of magnetic fiux is an electrical coil and the two flat parallel sides of the source of magnetic flux are opposite ends of the electrical coil, said electrical coil being divided midway between its flat ends into two equal parts which are spaced to allow access to the space between the magnetic reflecting surfaces of said first and second magnetic members.

4. The magnet of claim 2 wherein said source of magnetic flux is an electrical coil and the two flat parallel sides of the source of magnetic flux are opposite ends of the electrical coil, said electrical coil being divided midway between its ends into two equal parts, said electrical coil extending outwardly from the opening defined by the first, second, third and fourth magnetic members with the two parts of the electrical coil diverging to allow access to the space between the magnetic reflecting surfaces of said first and second magnetic members.

5. The magnet of claim 2 wherein said source of magnetic flux is first and second toroidal coils each having' an equal number of turns, and the two fiat parallel sides of the source of magnetic flux are opposite ends of each of the first and second coil-s, said first and second coils surrounding said third and fourth magnetic members with at least a portion of their opposite ends contacting the magnetic reflective surfaces of said first and second magnetic members, said first and second coils providing magnetic flux which is in opposite directions in the magnetic circuit and which is additive between the magnetic reflective surfaces of said first and second magnetic members.

6. The magnet of claim 2 wherein said source of magnetic flux is an electrical coil having a plurality of layers of turns formed of a conductor having a predetermined cross section, said electrical coil having an additional turn for each two half turns which are missing due to non-uniformi-ties in turns per layer due to conductor cross section and the disposition of the turns against the flat magnetic reflecting surfaces of said first and second magnetic members.

References Cited by the Examiner UNITED STATES PATENTS 3,056,070 9/1962 Nelson 317158 3,173,079 3/1965 McFee. 3,197,678 7/1965 Primas 317-158 OTHER REFERENCES High Magnetic Fields, The MIT. Press and John Wiley & Sons, Inc., New York, 1962. (An article by Betterton et al., pp. 348 and 349.)

BERNARD A. GILHEANY, Primary Examiner.

G. HARRIS, JR., Assistant Examiner. 

1. A CROSS FIELD ELECTROMAGNET COMPRISING TWO SPACED APART LEG MEMBERS, SAID LEG MEMBERS HAVING PARALLEL FACING SIDES, SAID LEG MEMBERS CONNECTED BY TWO PARALLEL YOKE MEMBERS TO FORM A CLOSED MAGNETIC CIRCUIT, A RECTANGULAR ELECTRICAL COIL WOUND ABOUT EACH OF SAID LEG MEMBERS AND IN CONTACT WITH EACH OF SAID YOKE MEMBERS, EACH OF SAID ELECTRICAL COILS HAVING A GERNERALLY RECTANGULAR CROSS-SECTION, SAID COILS BEING WOUND SO THAT THE MAGNETIC FLUX PRODUCED BY EACH COIL BETWEEN SAID YOKE MEMBERS IS ADDITIVE. 